1. Field of the Invention
The present invention relates to a display device, particularly, a technology effective in application to a display device in a system that a digital signal is transferred between driving circuits.
2. Description of Related Arts
A TFT (thin film transistor) type liquid crystal display module is widely used as a display device for a notebook personal computer and such. The liquid crystal display module comprises a liquid crystal display panel formed by holding a liquid layer between two transparent substrates different in size and a driving circuit for driving the liquid crystal display panel.
As such a liquid crystal display module, known has been one in a system that a digital signal (display data or a clock, for example) is inputted only to the top of the cascaded driving circuits while a digital signal passes through driving circuits to be sequentially transferred to the other driving circuits (referred to as a digital signal sequential transfer system), as described in Japanese Patent Laid-open No. Hei 06-13724 (Patent Reference 1), for example.
In the liquid crystal display device described in Patent Reference 1, a semiconductor integrated circuit device (IC) forming a driving circuit is directly mounted to a transparent substrate (a glassboard, for example) forming the liquid crystal display panel. A power source voltage for each of gate drivers is supplied from a power source circuit through power source wiring provided on a substrate of the liquid crystal display panel.
In the case of supplying the power source wiring on the transparent substrate forming the liquid crystal display panel with current only from one side, the larger the size of the liquid crystal display panel is, the more the resistance of the power source wiring on the transparent substrate forming the liquid crystal display panel increases. For example, a fall of the voltage due to the resistance of the power source wiring causes unevenness of the power source voltage supplied to each gate driver.
The unevenness of the supplied power source voltage in the respective gate drivers causes a change in on-resistance of a thin film transistor (TFT), and thereby, unevenness of an image voltage written in each pixel electrode (ITO 1), so that unevenness in display occurs.
Accordingly, in the case of a liquid crystal display panel as large as or more than 12.1 inches in dimension, electric current is fed to the power source wiring on the transparent substrate forming the liquid crystal display panel from the both sides, using a circuit board, to stabilize a power source voltage supplied to each gate driver.
On the other hand, there has been a problem that the liquid crystal display module is largely required to be lowered in cost while the liquid crystal display module using the above-mentioned digital signal sequential transfer system is difficult to be further lowered in cost since a circuit board is expensive.
The invention is to solve the problems in the conventional technology. An object of the invention is to provide a technology enabling the cost of a display device to be reduced more than the conventional one.
The above-mentioned and other objects and new characteristics of the invention will be disclosed on the basis of the description and attached drawings of the specification.